Power plants comprising a direct driven generator have several advantages, such as high mechanical reliability and low mechanical losses, due to the fact that there is no gear between the generator and a prime mover, e.g. a turbine, of the power plant. In many applications, a drawback of directly driven generators is that the mechanical size of the generator and thereby its weight can be quite big because the mechanical size is determined by the required maximum torque and the cooling of the generator. In a radial flux electrical machine, the maximum torque is proportional to the product of the air-gap radius, the area of the air-gap surface, the magnetic flux density (Tesla) in the air-gap and, the linear current density (Amperes/meter) in air-gap surface. Hence, without increasing the mechanical size of the electrical machine, the maximum torque can be increased by increasing the linear current density because the magnetic flux density cannot practically be increased any more when it is above the saturation point of iron. Increasing the linear current density increases, however, the resistive losses in a winding that produces the linear current density, and therefore the cooling of the said winding has to be made more effective.
The most effective method for cooling the winding of an electrical machine is a liquid cooling in which the cooling liquid is in contact or at least in close vicinity of electrical conductors of the winding. The liquid cooling of the winding is traditionally used in conjunction with large turbo-generators in which the electrical conductors of stator coils can be hollow so as to allow the cooling liquid to flow inside the electrical conductors. For example, the publication UA73661 discloses a liquid cooled stator of an electrical machine. The stator comprises a magnetic core with hydrogen cooling and a three-phase winding having hollow bars for the cooling liquid. Hydrogen pressure in the hydrogen cooling system of the magnetic core corresponds to liquid pressure in the liquid cooling system of the winding. A general inconvenience related to liquid cooled windings is that manufacturing and assembling of a liquid cooled winding is more complex and expensive than manufacturing and assembling of an air cooled winding.